The present invention relates to a method for producing a carrier for microorganisms and more particularly, to a method for producing a carrier for microorganisms which is packed in a device for biological reaction (bioreactor) used for treatment of water, production by fermentation or the like and capable of maintaining microorganisms in a high concentration.
In recent years, a technique for continuously performing a reaction by immobilizing microorganisms in a bioreactor has been often adopted. The technique includes, for example, a carrier binding which comprises immobilizing microorganisms onto a water-soluble carrier by an ion binding or covalent binding method, a crosslinking method which comprises strengthening cell walls or cell membranes of microorganisms using a reagent having two or more functional groups and at the same time, effecting crosslinking between the microorganisms, an inclusion method which comprises including microorganisms into a high molecular gel matrix or including them into semipermeable high molecular thin membrane microcapsules, etc.
The present invention is not directed to such methods for immobilizing microorganisms but relates to a method for producing the carrier for microorganisms in a method for adhering and immobilizing microorganisms to a carrier.
In old trickling filters, crushed stone, cokes, etc. are generally used as such an adhesive type carrier for microorganisms; in high speed trickling filters, synthetic resin plates, synthetic resin rings, etc. are used; and in rotary disc devices, synthetic resin plates and other activated charcoal, porous ceramics, naturally occurring inorganic particles, synthetic resin particles, etc. are used. Microorganisms are biologically adhered to these carriers. These carriers of adherence type have been used in bioreactors for treatment of water or production by fermentation in the form of fixed bed type, fluidized bed type or floating bed type. A concentration of microorganisms adhered varied depending upon carrier so that kind of carriers for microorganisms affects efficacy of bioreactors. In general, a specific surface area (m.sup.2 /m.sup.3) of carrier becomes an important factor; the larger the specific surface area, the larger the concentration of microorganisms adhered.
However, a carrier having a large specific surface area enables to keeping the microorganisms in a high concentration on one hand but on the other hand, silting occurs in a bioreactor due to growth of microorganisms, resulting in various problems such as reduction in efficiency of contacting substrate with microorganisms, reduction in contact efficiency upon supply of a gas containing oxygen, reduction in efficiency of contacting substrate with microorganisms due to an entrapping effect of generated gas, etc.
Accordingly, in order to avoid such problems, a specific surface area value of about 60 to 120 (m.sup.2 /m.sup.3) is generally adopted, taking as an example a carrier module obtained by laminating synthetic resin plates. A concentration of the microorganisms adhered in this case is merely approximately 2,000 to 3,000 mg/l, when calculated into a volume of exposure tank.
The present applicant has proposed a carrier for microorganisms produced from nonwoven fabrics, etc. which can be used for aerobic or anaerobic bioreactors and can maintain microorganisms in a high concentration 2 or 3 times that in the prior art methods described above (Published Unexamined Japanese Patent Application No. 61-149085).
However, it has become clear that the carrier for microorganisms produced from nonwoven fabrics encounters such a defect that could not be used as a carrier for packing in a commercially available bioreactor in the state shown in Published Unexamined Japanese Patent application No. 61-149085.
Firstly, it has been clarified that the carrier is strongly entangled with each other since it is made of fabrics such as nonwoven fabrics so that fluidization of the carrier becomes difficult. The present applicant has made investigations on coating the surface of the carrier with a film to avoid this problem. However, it has been noted that costs for the coating are extremely large and its commercialization is difficult.
Secondly, it has been noted that carriers produced from nonwoven fabrics or the like are swollen by immersion in water over long periods of time.
Thirdly, it has been noted that carriers produced from nonwoven fabrics or the like have a poor compression strength and distortion of the carriers is unavoidable. Fabrics are originally fragile in strength. In addition, particularly in commercial plant using a large amount of carriers, the carriers are scaled up in the height direction so that a force of compressing onto the carriers increases and the carrier distortion is unavoidable. For this reason, withdrawal of a gas supplied or a gas generated is seriously worsened, resulting in reducing a contact efficiency between microorganisms and substrate. As a result, these methods involve a defect that efficiency of bioreactors becomes low.
The present invention aims at providing a method for producing a carrier for microorganisms capable of eliminating the prior art defects described above, maintaining microorganisms in a high concentration and being sufficiently used for bioreactors of commercial scale.